U.S. Pat. No. 3,144,546 discloses water heating via electrodes. U.S. Pat. No. 3,144,546, describes the electric circuit thereof as follows:                Preferably two separate electrodes are used, each connected to a separate lead of a three-wire electric current supply system such as is commonly used with 220-volt house wiring circuits. When a three-wire 220-volt circuit is used, it is preferred that the housing be connected to a lead, usually called the ground wire, so that there is a potential of volts between the housing and each of the electrodes and a potential of 220 volts between the electrode.        
However, water heating by electrodes practically is not used for residential boilers, for supplying the water thereof for washing, since being regarded not safe. For instance, https://en.wikipedia.org/wiki/Electric_heating describes the following: “Electrode heater: With an electrode heater, there is no wire-wound resistance and the liquid itself acts as the resistance. This has potential hazards, so the regulations governing electrode heaters are strict.”
The reason for the potential hazards may be explained according to U.S. Pat. No. 3,144,546, describing the electric circuit thereof (mentioned above).
FIG. 1 depicts the electric circuit of the prior art electrode heating.
Electrodes 18 and 20 are immersed in the water. The electric grid supply 26 is connected to electrodes 18 and 20.
FIG. 2 depicts the problem of the prior art electric circuit.
According to U.S. Pat. No. 3,144,546, the contained water is electrified by the 220 volts potential of electric grid supply 26, and carries it to the ground 24, and during washing, the current flows through human user 36. This produces an electric leak, and thus, the residual-current device, does not allow this electric flow, and cuts off the current.